In general, the most ideal receiver architecture for an integrated circuit from a bill-of-material point of view is usually a direct down conversion (DDC) architecture. However, in practice, there are several issues that often prohibit the practical design of integrated circuit implementations that use DDC architectures. These issues typically include noise from the DC offset voltage and 1/f noise from baseband circuitry located on the integrated circuit. In mobile applications, such as with cellular phones, the DC offset voltage is a time varying entity which makes its cancellation a very difficult task. In other applications where mobility is not a concern, such as with satellite receivers, the DC offset voltage can be stored and cancelled, such as through the use of external storage capacitors,. However, 1/f noise is still an issue and often degrades CMOS satellite tuners that use a DDC architecture.
Conventional home satellite television systems utilize a fixed dish antenna to receive satellite communications. After receiving the satellite signal, the dish antenna circuitry sends a satellite spectrum signal to a satellite receiver or set-top box that is often located near a television through which the viewer desires to watch the satellite programming. This satellite receiver uses receive path circuitry to tune the program channel that was selected by the user. Throughout the world, the satellite channel spectrum sent to the set-top box is often structured to include 32 transponder channels between 950 MHz and 2150 MHz with each transponder channel carrying a number of different program channels. Each transponder will typically transmit multiple program channels that are time-multiplexed on one carrier signal. Alternatively, the multiple program channels may be frequency multiplexed within the output of each transponder. The total number of received program channels considering all the transponders together is typically well over 300 program channels.
Conventional architectures for set-top box satellite receivers include low intermediate-frequency (IF) architectures and DDC architectures. Low-IF architectures utilize two mixing frequencies. The first mixing frequency is designed to be a variable frequency that is used to mix the selected satellite transponder channel to a pre-selected IF frequency that is close to DC. And the second mixing frequency is designed to be the low-IF frequency that is used to mix the satellite spectrum to DC. Direct down conversion (DDC) architectures utilize a single mixing frequency. This mixing frequency is designed to be a variable frequency that is used to mix the selected satellite transponder channel directly to DC.
As indicated above, DDC architectures are desirable due to the efficiencies they provide. DDC architectures, however, suffer from disadvantages such as susceptibility to DC noise, 1/f noise and I/Q path imbalances. DDC architectures also often require narrow-band PLLs to provide mixing frequencies, and implementations of such narrow-band PLLs typically utilize LC-based voltage controlled oscillators (VCOs). Low-IF architectures, like DDC architectures, also typically require the use of such narrow-band PLLs with LC-based VCOs. Such LC-based VCOs are often difficult to tune over wide frequency ranges and often are prone to magnetically pick up any magnetically radiated noise. In addition, interference problems arise because the center frequency for the selected transponder channel and the DDC mixing signal are typically at the same frequency or are very close in frequency. To solve this interference problem, some systems have implemented receivers where the DDC mixing frequency is double (or half) of what the required frequency is, and at the mixer input, a divider (or doubler) translates the DDC mixing signal into the wanted frequency. Furthermore, where two tuners are desired on the same integrated circuit, two DDC receivers, as well as two low-IF receivers, will have a tendency to interfere with each other, and their VCOs also have a tendency to inter-lock into one another, particularly where the selected transponder channels for each tuner are close together.